Glazing produced from compositions comprising transparent thermoplastic polymers, such as e.g. polycarbonate, offers many advantages over conventional glazing of glass for the vehicle sector and for buildings. These include e.g. increased fracture-proof properties or saving in weight, which in the case of automobile glazing makes possible a higher safety of passengers in the event of traffic accidents and a lower fuel consumption. Finally, transparent materials which contain transparent thermoplastic polymers allow a considerably greater freedom of design due to the simpler formability.
A disadvantage is, however, that the high transparency to heat (i.e. transparency to IR radiation) of transparent thermoplastic polymers in sunlight leads to an undesirable heating inside vehicles and buildings. The increased temperatures in the inside reduce the comfort for the passengers or occupants and can result in increased demands on the air-conditioning, which in turn increase energy consumption and in this way cancel out the positive effects again. In order nevertheless to take into account the requirement of a low energy consumption combined with a high passenger comfort, panes which are equipped with appropriate heat protection, which is ensured over a long period of use, are necessary. This applies in particular to the automobile sector.
As has been known for a long time, the majority of solar energy falls to the range of the near infra-red (NIR) between 750 nm and 2500 nm, in addition to the visible range of light between 400 nm and 750 nm. Penetrating solar radiation e.g. is absorbed inside an automobile and emitted as long wavelength thermal radiation with a wavelength of from 5 μm to 15 μm. Since in this range conventional glazing materials—in particular thermoplastic polymers which are transparent in the visible range—are not transparent, the thermal radiation cannot radiate outwards. A greenhouse effect is obtained and the interior heats up. In order to keep this effect as low as possible, the transmission of the glazing in the NIR should therefore be minimized as far as possible. Conventional transparent thermoplastic polymers, such as e.g. polycarbonate, however, are transparent both in the visible range and in the NIR.
Additives e.g. which have the lowest possible transparency in the NIR without adversely influencing the transparency in the visible range of the spectrum are therefore required.
Among the transparent thermoplastics, polymers based on polymethyl methacrylate (PMMA) and polycarbonate are particularly suitable for use as glazing material. Due to the high toughness, polycarbonate in particular has a very good profile of properties for such intended uses.
In order to impart to these plastics heat-absorbing properties, corresponding infra-red absorbers are therefore employed as additives. In particular, IR absorber systems which have a broad absorption spectrum in the NIR range (near infra-red, 750 nm-2500 nm) with a simultaneously low absorption in the visible range (low inherent colour) are of interest for this. The corresponding polymer compositions should moreover have a high heat stability and an excellent light stability.
A large number of IR absorbers based on organic or inorganic materials which can be employed in transparent thermoplastics are known. A selection of such materials is described e.g. in J. Fabian, H. Nakazumi, H. Matsuoka, Chem. Rev. 92, 1197 (1992), in U.S. Pat. No. 5,712,332 or JP-A 06240146.
Nevertheless, IR-absorbing additives, in particular those based on organic materials, often have the disadvantage that they have a low stability towards exposure to heat or irradiation. Thus, many of these additives are not sufficiently stable to heat to be able to be incorporated into transparent thermoplastics, since temperatures up to 350° C. are required during their processing.
Furthermore, the glazing is often exposed to temperatures of more than 50° C. over relatively long periods of time during use, due to the solar irradiation, which can lead to decomposition or to degradation of the IR-absorbing additives (IR absorbers).
Although IR-absorbing additives based on inorganic materials are often significantly more stable compared with organic additives, these also show a significant degradation in the absorber performance over time, in particular under high exposure to heat, i.e. at temperatures >50° C. for a relatively long period of time. In the case of buildings and automobile glazing, total exposure times of days, weeks or even years are to be taken into account here. The fact that IR absorbers based on inorganic materials show unstable properties under exposure to heat was surprising, since the person skilled in the art in principle assumes that inorganic systems based on metal oxide or boride have a high heat stability.
Materials based on finely divided borides, such as e.g. lanthanum hexaboride, have become established as inorganic IR absorbers, since they have a broad absorption band. Such borides based on La, Ce, Pr, Nd, Tb, Dy, Ho, Y, Sm, Eu, Er, Tm, Yb, Lu, Sr, Ti, Zr, Hf, V, Ta, Cr, Mo, W and Ca are described e.g. in DE 10392543 or EP 1 559 743.
IR absorbing additives from the group of tungsten compounds which have a lower inherent absorption in the visible spectral range compared with the inorganic boride-based IR absorbers known from the prior art are furthermore known, in particular zinc-doped tungsten compounds with an increased long-term stability preferably being used.
In the context of the invention, inorganic IR absorbers in particular are stabilized by the present stabilizer combination. Among the inorganic IR absorbers, in particular borides, here especially lanthanum hexaboride, tungstates, in this case in particular caesium tungstates and zinc-doped caesium tungstates, tin oxides, in particular indium tin oxide (ITO), and tin-doped antimony oxide (ATO) are preferred.
The preparation and the use of these absorbers in thermoplastic materials are described, for example, in H. Takeda, K. Adachi, J. Am. Ceram. Soc. 90, 4059-4061, (2007), WO 2005037932, JP 2006219662, JP 2008024902, JP 2008150548, WO 2009/059901 and JP 2008214596.
It was moreover known to use heat stabilizers, such as, for example, phosphites, hindered phenols, aromatic, aliphatic or aliphatic-aromatic phosphines, lactones, thioethers and hindered amines (HALS, hindered amine light stabilizers) in thermoplastic materials to improve the processing properties.
WO-A 01/18101 discloses moulding compositions comprising a thermoplastic and a phthalo- or naphthalocyanine dyestuff which can contain antioxidants, such as phosphites, hindered phenols, aromatic, aliphatic or mixed phosphines, lactones, thioethers and hindered amines to improve the processing stability.
In all the thermoplastic compositions with IR absorbers published to date, the heat stabilizer serves exclusively, however, to stabilize the particular polymer matrix—in particular during processing. By using these systems, the yellow coloration of the polycarbonate after exposure to light, as described in EP 1266931, can thus be limited.
In the case of all the abovementioned IR absorbers, also in combination with in each case conventional stabilizers, it has been found, however, that the long-term stability under exposure to heat such as occurs in the life cycle of the materials is inadequate.
This applies not only to the organic but also to the inorganic IR absorbers in a transparent thermoplastic polymer matrix, so that after storage of the corresponding polymer compositions, such as e.g. a polycarbonate composition, in heat at elevated temperature the absorption in the IR range decreases significantly.
For use of the compositions in the glazing sector, in particular for automobile glazing, however, it is absolutely essential that the corresponding IR-absorbing polymer compositions have a long-term stability to higher temperatures. Higher temperatures mean e.g. temperatures which an article of polycarbonate can assume under intensive solar irradiation (e.g. 50° C.-110° C.). It must furthermore be ensured that the composition can be processed under conventional process conditions, without the IR-absorbing properties already being reduced as a result.
It was known from DE 10 2009 058200 that IR absorbers based on caesium tungstate can be stabilized in the thermoplastic matrix by addition of triphenylphosphine. Nevertheless, a marked degradation of the stabilizer performance in a long-term experiment at a temperature of 120° C. is also found with this stabilizer.